Js variable scope issues, js Variables

Js variable scope issues, js Variables

During job interviews, we often encounter the following questions:

Var a = "Hello"; function B () {alert (a); var a = "World"; alert (a) ;} B ();

The correct running result is: undefined, world

Why is this result?

First, we need to clarify the following points:

The above question will be changed to the following after js pre-Compilation:

Var a = undefined; a = "Hello"; function B () {var a = undefined; alert (a); a = "World"; alert ();} B ();

** Therefore, when alert is used for the first time, the var definition of variable a is found in the scope of function B, and the value is undefined;
When the second alert operation is performed, variable a in scope B is assigned the world value, so the value is world **